Photovoltaic (PV) systems are systems that convert light into electricity. All photovoltaic systems share a few common parts. All photovoltaic systems include light-harvesting function, a charge-separating function, charge-transporting function, and a charge collecting function.
In the past several years, organic-inorganic metal halide perovskites (ABX3, A=CH3NH3+, (NH2)2CH+, B=Pb2+, Sn2+, X=Cl−, Br−, I−) have risen to be a class of promising new photovoltaic materials due to their attractive merits, such as solution processability, low cost of material precursors and ease of device fabrication.1-3 Among the class of hybrid perovskites, methylammonium lead iodide (CH3NH3PbI3) represents an outstanding light absorber (1.5×104/cm at 550 nm)4,5 with superior photovoltaic properties such as ease of free carrier generations6, long carrier diffusion lengths7,8 even with its moderate mobilities,9 and the surprisingly long carrier lifetimes7,10, and has attracted extensive attention for its phenomenal photovoltaic performance, as well as other emerging properties including ferroelectrics and nano-lasing.11-23 Although CH3NH3PbI3 owns such prominences, it suffers from several inherent problems, including lead toxicity, current-voltage hysteresis, and low stability in humidity.24-27 According to perovskite crystal structures and previous studies, the first step of structural degradation due to moisture involves the formation of hydrated PbX64− intermediate and removal of the methylammonium from the sub-lattice structure, this produces a large amount of charge imbalance and the resultant self-repelling PbI3− lattice collapses to generate PbI2. In order to tackle this problem, modifications of chemical composition in the perovskite active layer have been practiced by introducing dopants and new chemical moieties geared towards moisture tolerance.4,28,29 Successful examples of enhancing moisture stability of perovskites was previously realized through partially replacing the halides with pseudohalides such as SCN− or partially replacing the CH3NH3+ with butylammonium, so as to convert the three-dimensional perovskite structure to a Ruddlesden-Popper type, two-dimensional perovskite structures.4,30-33 However, the resulting layered structure hinders charge transport through structurally confining photocarriers in 2-D inorganic sheets of the materials.33, 34 Thus, it is imperative to explore chemical pathways that retain the 3-dimensionality of the perovskite structure and good photo-absorption while still enhancing the chemical stability of the entire structure to battle the invasion of water molecules.